Being loose can refer to a person who is relaxed and casual. Antonyms for this expression might include rigid, tense, or uptight. If someone is described as being loose in their morals or behavior, antonyms could include virtuous, chaste, or disciplined. Being loose in the sense of not fitting properly might be describable with words like snug, tight, or secure. In terms of communication, someone who is loose with their words might be contrasted with someone who is measured or careful in their speech. In any case, antonyms for being loose often carry connotations of constraint, restraint, or precision.